All motors and engines that perform useful work consume energy in one form or another. The energy used by an engine may be used in the same form as provided by the energy source, or may be utilized in a form converted from the energy source. For instance, even though some devices respond directly to sunlight, solar energy usually is harnessed by converting sunlight energy into electrical energy, and then utilizing the electrical energy to drive the engine. Some energy sources, such as solar, hydro and geothermal, are practically inexhaustible, while all more commonly used energy sources, particularly fossil fuels and derivatives thereof, are irreversibly depleted when used. The cost of putting a load into Earth orbit is about $40,000 per pound; therefore, a gallon of fossil fuel costs about $320,000 in space. As an example, while energy in many satellites is provided by the sun, most space vehicles utilize depleting energy sources, such as hydrogen and oxygen, to perform necessary work, such as generating electricity by means of fuel cells. To derive one megawatt of power from the sun requires a solar collector of 10,000 square meters (about twice the size of a football field). Similarly, in extremely remote areas having harsh topography, such as the Ural mountains or the Himalayas, the provision of reliable energy sources to perform work in isolated instances is formidable. Although pipes to supply fluidic fuel or wires to supply electrical power may be utilized, in many cases this is entirely impractical. Ocean-going vessels carry expensive fuel that accounts for a significant fraction of their tonnage. Equipment operating in space has an additional disadvantage related to the inefficiency of heat rejection because the only available technique is radiative heat rejection, which requires very large areas, just as do solar collectors.